The present invention relates to providing an optical pickup mounted on a compact disk player, and particularly to providing a fixing structure for a beam splitter in an optical pickup for reducing the optical axis deviation of the light reflected from the light receiving element due to the inclination of the beam splitter.
FIG. 5 illustrates the skeleton of an optical device using a conventional optical pickup. In the optical pickup, the laser beam is reflected toward the left by means of light-distribution division surface 3a (which is drawn as the half-mirror in FIG. 5) of beam splitter 3. This reflected light is made into a parallel beam by collimator lens 4, reflected at a right-angle by means of the total reflection prism 5, and converged on the recording surface of disk D. The beam reflected by the recording surface of disk D returns through objective lens 6, total prism 5, and collimator lens 4 along the original path, and passes through light-distribution division surface (half mirror) 3a to be guided by concave lens 7. The focal point of the light returned through concave lens 7 is extended and guided to pin photodiode 8 and the reflected beam is detected. Also, a cylindrical lens is generally mounted along with concave lens 7, but it is omitted in the drawings. The information reproduction is performed in such a manner that the light quantity of the modulated reflected beam is photodetected by means of the pit formed on the recording of the optical disk. In the prior art, the mounting of beam splitter 3 is performed in the manner that both ends of beam splitter 3 are attached by means of an adhesion agent to chassis 9 of the optical or they are pressed by means of a spring.
In the optical apparatus of the optical pickup, light-distribution division surface 3a of beam splitter 3 is positioned at 45.degree. with respect to both the optical axis of light reflected from recording surface D and returned from objective lens 6 and the axis of light emitted from semiconductor laser 1. But, because of the dimension tolerance of beam splitter attaching portion of the chassis, the tolerance of the degree of parallelism between the attaching portion of beam splitter 3 and light-distribution division surface 3a, the thickness of the adhesion agent, and the thermal expansion or the thermal contraction of beam splitter 3, the angle of the optical axis of the reflected light and the emitted light with respect to the light-distribution division surface 3a is not accurately set at 45.degree..
The light reflected by the optical disk penetrates the Q point of light-distribution division surface 3a and is focused on the converging light point P. The distance from the Q point to the P point is the same as that from the light-emitting point O to the Q point. Also, the focus distance of the collimating lens 4 converged on the P point is extended to the pin photodiode 8 by means of concave lens 7. If the position of light-distribution division surface 3a deviates from the angle of 45.degree., the position of the converging light point P is deviated upwardly or downwardly in FIG. 5 and the detecting point of pin photodiode 8 is moved from the original position, with the result that the lower of the signal reproduction or the tracking error detecting accuracy is cancelled.
FIG. 6 represents the deviation of the converging point P (the converging point in the case that the concave lens is not used when light-distribution division surface 3a of beam splitter 3 is further inclined by only the angle .theta. (dotted line 3a') from the original angle (solid line 3a) of 45.degree.. Optical axis LB1 of the laser beam produced at the light-emitting center of semiconductor laser 1 is reflected in the perpendicular direction by means of light-distribution division surface 3a to give laser beam LB2 (LB2 shows the optical axis), if light-distribution division surface 3a of beam slitter 3 is accurately set at 45.degree.. And, the reflected light from this laser beam LB2 is again reflected by the recording surface of the disk D, returned to its original path, and penetrates the Q point of light-distribution division surface 3a to be focused at the P point separated from the Q point. But, if light-distribution division surface 3a of beam splitter 3 is further inclined by the angle .theta..degree. as shown by the dotted line 3a' in FIG. 6, the incident angle formed between the light-emitting optical axis LB1 and light-distribution division surface 3a' of the dotted line is set at 45.degree.+.theta. as represented in FIG. 6. Accordingly, as shown by the dotted line LB2' the angle formed between the reflected optical axis LB2' of the laser beam and light-distribution division surface 3a' is 45.degree.+.theta.. The beam including this reflected optical axis LB2' is again reflected from the recording surface of the disk D, returning along the original path and penetrating the Q point on light-distribution division surface 3a and is focused at the P1 point spaced by more than the distance l from the Q point.
Since light-distribution division surface 3a is inclined from the position of the solid line 3a to the position of dotted line 3a', the angle between LB3 and LB3', the optical axis LB3 being moved to LB3' by the additional inclination of 3a to 3a', is found as follows. In FIG. 6, since the reflecting angle between dotted line 3a' and optical axis LB2' of the reflected beam is 45.degree.+.theta., said angle is found by adding the angle .theta. to the reflected angle and subtracting the 45.degree. therefrom. That is to say, the angle between the transmitted optical axis LB3 and LB3' as follows; EQU (45.degree.+.theta.)+.theta.-45.degree.=2.theta..
If light-distribution division surface 3a is inclined by only the angle .theta., the incident angle and the reflected angle of the light beam with respect to said surface are accumulated. Transmitted optical axis LB3' is inclined at the angle 2.theta. with respect to LB3. Thus, if a concave lens is not used, the deviation .DELTA.x of the converged light point from P to P' is as follows; EQU .DELTA.x=[1] l sin 2.theta..
Also, if beam splitter 3 is inclined at only the angle .theta. toward the clockwise direction (the reverse direction of FIG. 6) about the Q point, the deviation of the converged light point P is l sin 2.theta. in accordance with the above calculation but in this case, the converged light point P is moved downward in FIG. 6.
As described above, if light-distribution division surface 3a of beam splitter 3 is further inclined by the angle .theta. from the original position, transmitted optical axis LB3' is inclined by the angle 2.theta. and converged light point P is moved by .DELTA.x from its original position. This deviation caused the detection point of pin photodiode 8 to be moved from the original point and the detecting accuracy to be lowered.